August 6, 2009

Aesop’s Crow Fable May Indeed Be True

Aesop's fables, which include the goose that laid the golden egg to the notorious race between the tortoise and the hare, are known for their moral lessons rather than their literal truth.

But a new study finds at least one such tale might actually be rooted in fact: the fable of "the crow and the pitcher."

The story is the tale of a bird that finds a pitcher with the water level too low for him to reach, causing the crow to raise the water level by dropping rocks into the pitcher. Depending on the telling of the story, the moral is that goals can be accomplished one small step at a time, or that necessity is the mother of invention.

But scientists say that some relatives of crows, known as rooks, actually employ this same stone-dropping tactic to get at a floating worm.

The study, led by Christopher Bird of Cambridge University, finds that birds belonging to the corvid (or crow) family are able to solve complex problems using tools and can easily master the same technique demonstrated in Aesop's fable.

"Corvids are remarkably intelligent, and in many ways rival the great apes in their physical intelligence and ability to solve problems. The only other animal known to complete a similar task is the orang-utan," said Bird.

"This is remarkable considering their brain is so different to the great apes'. Although it has been speculated in folklore, empirical tests are needed to examine the extent of their intelligence and how they solve problems.

In the first part of the study, Bird and a colleague exposed rooks to a 6-inch-tall clear plastic tube containing water, and placed a worm that floated on the surface. The researchers varied the height of the water, and found the four rooks used stones to raise the water level to reach the worm.

The clever birds were also highly accurate in their ability, adding the exact number of stones needed to raise the water level to the necessary height.

Two of the birds were successful on their first try to raise the height of the water to a level at which the worm floating on top could be reached while the other two birds needed a second try.

Additionally, rather than attempting to reach the worm after each stone was dropped, they apparently estimated the number needed from the outset and waited until the appropriate water level was reached before dipping their beaks into the tube.

In the second experiment the rooks were presented with stones that varied in size. The rooks selected larger stones over smaller ones (although not immediately). The scientists speculate that the birds learned rapidly that the larger stones displaced more water and they were therefore able to obtain the reward more quickly than using small stones.

In the third experiment, the rooks were observed to recognize that sawdust could not be manipulated in the same manner as water. Therefore when presented with the choice between a tube half-filled with either sawdust or water, rooks dropped the pebbles into the tube containing water and not the sawdust.

Although the study demonstrates the flexible nature of tool use in rooks, they are not believed to use tools in the wild.

"Wild tool use appears to be dependent on motivation," Bird said.

"Rooks do not use tools in the wild because they do not need to, not because they can't. They have access to other food that can be acquired without using tools."

That fits nicely with the moral of Aesop's fable as demonstrated by the crow: "Necessity is the mother of invention."

Results of experiments with three birds were published online August 6 in the journal Current Biology.

In accompanying commentary, Alex Taylor and Russell Gray of the University of Auckland in New Zealand pointed out that previous experiments had shown the rooks dropped a single stone into a tube to release food at the bottom. Perhaps they were just repeating the strategy when they saw the tube in the new experiment, the pair speculated.

However, Bird said there is more to it than that, noting that the rooks dropped several stones rather than just one before reaching for the worm. Furthermore, they reached for it at the top, rather than the bottom, of the tube.

Aesop's crow might have actually been a rook, the scientists concluded, since both types of birds were known as crows in the past.